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Predicted Producing Ability (PPA)

Modified on 2007/12/29 14:25 by geneder Categorized as Uncategorized
Predicted Producing Ability or PPA is calculated by the USDA (United States Department of Agriculture) as part of the national genetic evaluations for yield and management traits. It is calculated from DHI data and its purpose is to predict future production. PPA offers many advantages to producers in identifying cull does. The USDA animal model genetic evaluation system uses statistical procedures to evaluate the impact of different factors on production records of a single doe. The factors are effects of management group (herdmates to the doe), the additive or heritable part of genetic differences between does, effects in common with other paternal half sisters in the same herd, permanent environmental effects that act on all lactations of one doe, and everything that is left over. Published genetic evaluations are called PTA (Predicted Transmitting Ability), and are half of the additive genetic value for a dam or sire. Genetic merit for each trait is halved because the purpose of PTA is to predict the part each parent transmits to progeny.

PPA has a different purpose from PTA. PPA predicts how much milk a doe herself will produce in subsequent lactations compared to her herdmates. PPA includes the entire additive genetic component rather than half, because all of those genes will affect doe performance. PPA also includes two other factors in the animal model that repeat from one lactation to another, effects common to all paternal half sisters of the doe in her herd, and the permanent environmental effects specific to the doe. One very valuable feature of PPA is that the USDA system combines genetic information from all other relatives of a doe that can be found in the national database. The advantage of this feature is considerable. PPA is the best prediction of future production available anywhere in the country. Unfortunately, we have not made very good use of PPA for within herd management.

PPA is the most complete and accurate predictor of future production that is currently calculated. The disadvantage is that PPA values are not widely available for use by dairy farmers. Unlike the Ratings system, which is re-evaluated following each monthly test, USDA calculates PPA values four times per year. ERPA's are calculated twice a year, which does affect their utility as a culling guide. The time lag between test day and a new PPA means that the information is not as timely as the Ratings system. Also, PPA must be transferred from the USDA computer system to one of the DHI processing centers before being merged into DHI records. PPA is only available on does that pass USDA edits for parentage identification, birth date and so forth.

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